Hydrocarbons may exist in reservoirs in subterranean rock formations. Generally, to produce the hydrocarbons from the formation, a wellbore is drilled in the formation and hydrocarbons travel from the formation to the wellbore through pores in the formation. The better the pores in the formation are connected (i.e. permeability), the better the hydrocarbon production. Some wells are poor producers from the beginning, whereas other wells produced satisfactorily only to decline into a poor producing well. Poor hydrocarbon production is commonly associated with decreased permeability due to plugged pores.
Hydrocarbon recovery stimulation techniques may be employed to improve the permeability of hydrocarbon-bearing formations. One such technique is fracturing, namely hydraulic fracturing. Hydraulic fracturing uses pump rate and hydraulic pressure to fracture or crack a subterranean formation thereby improving permeability of and production from the formation to the wellbore. To avoid “healing” of the formation after fracturing conditions are reduced or removed, a proppant, which is highly permeable relative to formation permeability, may be pumped downhole and into the fracture to prop it open. Since the proppant is highly permeability, it may provide a path for hydrocarbon flow. When an acid is used in the fracturing fluid to increase or restore permeability to the formation, the treatment term is “acid fracturing” or “acid frac”.
The fracturing fluid may be a thickened aqueous fluid. Traditionally, aqueous fracturing fluids have had their viscosities increased by incorporating hydratable polymers therein (e.g. polysaccharides), where some polymers may be crosslinked to increase viscosity even further. When aqueous fluids incorporating hydratable polymers are used downhole, the polymer may accumulate on and within the formation to form a polymeric filter cake. The formation may be damaged (e.g. plugged pores) when these polymeric filter cakes are incompletely removed prior to hydrocarbon production, which in turn may inhibit hydrocarbon production.
Non-polymeric viscoelastic surfactants (VES) are an alternative agent for thickening aqueous treating fluids. VES gelled aqueous fluids may exhibit very high viscosity at very low shear rates and under static conditions, which makes them an excellent choice for treating fluids such as fracturing fluids. Furthermore, some VES gelled fluids may be easier to remove from the formation, which may lead to less damage as compared to the polymeric-based fluids.
The viscosity of VES gelled fluids may be temperature dependent—the higher the temperature (to a point) the more viscous the fluid becomes. This temperature dependence may be beneficial in land-based operations where the earth's heat may warm the fluid or keep it warm. In offshore operations, however, the water temperature may be less than the surface temperature, and the temperature in deeper waters most certainly is less than the surface temperature. The viscosity of VES gelled fluids may be adversely affected in such cold conditions. For instance, less viscous fluids may have a diminished solids carrying capacity and/or fracturing capability.
Thus, it would be desirable if methods could be devised to enable VES gelled treating fluids to have higher viscosities in low temperature conditions.